Here in America “good on you” is not a commonly used phrase if it’s meant to mean “all credit to you”…instead we would say (if we were congratulating someone for example to give them credit or appreciation)….

“hey! you did it…well, good for you.”

or

“well good for you, you finished high school.”

never would we say…

“well good on you, you finished high school.”

So, in the phrase you mention, “good on you” the word “on” is a preposition and in American English would mean “to be supported by” …like for example if I were to say…

“That jacket looks good on you.” then the phrase makes sense since you are “wearing” or “supporting” the jacket…

in other words, it’s “on” you.

Like I said, maybe in the U.K. or perhaps elsewhere, “good on you” takes on an entirely different and logical meaning.

alternatively, if used with a sarcastic tone, it can indicate the opposite(someone has done something stupid)